In the field of communications, and more particularly wireless communications, a transmitter power amplifier generates a communication signal at a power level sufficient for that communication signal to be received at an intended receiver. To the extent that the power amplifier does not have a precisely linear transfer function, the communication signal will include some distortion. This distortion will adversely impact the ability of the receiver to correctly recover the information being conveyed. In digital communications, the adverse impact worsens as communication links attempt to operate at higher modulation orders and lower signal-to-noise ratios.
Moreover, this distortion introduces spectral regrowth into the communication signal. As more and more users vie for the limited electromagnetic spectrum, governmental regulations and system constraints each require communication signals to occupy as little of the spectrum as is necessary for the information being conveyed. Unfortunately, the distortion caused by power amplifier nonlinearities invariably causes the communication signal spectrum to increase.
One historical solution to the power amplifier nonlinearity problem has been to use high quality components or components otherwise restricted to operate only in a limited highly linear range of operation for the power amplifier to minimize spectral regrowth and distortion. Unfortunately, high quality components are too expensive, especially for mass market or otherwise high volume applications. Techniques that restrict operation to highly linear ranges of component operation tend to suffer from inefficient power use and are thus unsuitable for portable applications.
A more modern solution to the power amplifier nonlinearity problem has been to incorporate a power amplifier linearizer into the transmitter. In a typical digital communication application, the linearizer is implemented digitally through the use of look-up tables located upstream of the power amplifier, prior to analog conversion. The linearizer attempts to apply a particular transfer function to the signal it processes. The transfer function is desirably proportional to the inverse of the power amplifier transfer function. Thus, the linearizer predistorts a modulated signal causing the resulting communication signal, that will invariably be distorted by a power amplifier, to be as nearly perfectly linear as possible.
Several conventional techniques are known to those skilled in the art for determining a transfer function to be implemented by a linearizer and generating appropriate data to populate the look-up-tables used by the linearizer. In a typical conventional technique, the transmitter locally monitors and analyzes the communication signal being generated by its power amplifier to determine the nature of the distortion and to track any changes in the distortion. In order to do this, demodulation circuitry is supplied in the transmitter to locally demodulate the communication signal and processing circuitry is supplied in the transmitter to locally analyze the demodulated signal, characterize the distortion, out-of-band energy, or the like, and generate the data used to populate linearizer look-up-tables.
Such conventional linearization techniques suffer from several problems. One problem is that the conventional techniques do not allow the communication system to maintain positive control over spectrum use. Typically, a sample unit of a new radio design is tested in friendly, well-controlled environmental conditions for compliance with a spectral template imposed by governmental regulations. During manufacturing, additional samples may occasionally, although typically rarely, be tested for spectral compliance. This procedure allows numerous opportunities for non-compliant radios to be used by the general public. For example, some radio designs may be non-compliant when used over a range of environmental conditions. In accordance with other radio designs, some radios may be compliant, while others are non-compliant simply due to the vagaries of manufacturing processes. Moreover, some radios may have been compliant at one point in time, but become non-compliant due to component aging. Furthermore, unscrupulous manufacturers of radios may select for testing only those radios likely to prove themselves compliant, may substitute lower quality components for inclusion in manufacturing process when spectral compliance testing is unlikely, and the like. Consequently, many opportunities exist for non-compliant radios to be used by members of the general public.
The use of non-compliant radios is undesirable because it leads to increased interference levels. Increased interference levels diminish the ability of a provider of communication services to provide, and gain revenues from, communication services. Non-compliant radios are also undesirable simply because they are in violation of governmental regulations. Maintaining more positive control would be desirable to communication service providers because they could then insure that their users were using spectrally efficient radios that would minimally interfere with the communication services provided to other users. More communication services could then be provided to more users because interference levels would be diminished, and revenues could increase.
Conventional linearization techniques suffer from additional problems. For example, conventional techniques drive up the complexity and costs of providing communication services, particularly in point to multipoint (PTM) communication systems. A conventional cellular voice communication system is one example of a PTM communication system. In PTM systems, a large number of user radios typically communicate with a single hub radio. The use of conventional linearization techniques at the hub radio may not increase costs an intolerable amount when viewed from a system-wide perspective because those costs are incurred at relatively few sites. On the other hand, the use of conventional linearization techniques in user radios imposes intolerable costs on a communication system because of the large number of user radios that communicate with each hub radio. This cost problem is exacerbated by what has become a traditional business practice where the communication system provider heavily subsidizes the cost of user radios.
These problems are made worse when the user radios are portable. The inclusion of additional demodulation and processing circuitry is highly undesirable in portable radios because these items increase power consumption, weight, and physical radio size.
Another problem with some conventional linearization techniques is that the transmitters are required to transmit predetermined training sequences in order for the local linearization analysis circuitry to determine a suitable linearizer transfer function. The predetermined training sequences are transmitted in lieu of user payload data for which revenues may be generated. Consequently, these conventional linearization techniques diminish a communication service provider's ability to provide communication services to users and revenues that might otherwise be generated.